Power systems often include a power converter that is configured to convert an input power into a suitable power for application to a load, such as a generator, motor, electrical grid, or other suitable load. For instance, a power generation system, such as a wind generation system, can include a power converter for converting alternating current power generated at the generator into alternating current power at a grid frequency (e.g. 50/60 Hz) for application to a utility grid. An exemplary power generation system can generate AC power using a wind driven doubly fed induction generator (DFIG). A power converter can regulate the flow of electrical power between the DFIG and the grid.
Increased power DFIG systems can include a power converter that has multiple bridge circuits connected in parallel for each phase of the DFIG. Connecting multiple bridge circuits, such as H-bridge circuits, in parallel can provide for increased output power capability of the DFIG system. Each bridge circuit can include a plurality of switching elements (e.g. insulated gate bipolar transistors (Gifts)) driven by control commands. The control commands can control pulse-width-modulation (PWM) of the switching elements to provide a desired output of the power converter.
The switching elements in the parallel bridge circuits can be controlled according to various switching patterns. In an interleaved switching pattern, the switching elements in the parallel bridge circuits are switched in a manner out of phase with one another, such as 90° out of phase with one another. In a non-interleaved switching pattern, the switching elements in the parallel bridge circuits are switched in a nearly synchronous manner (i.e. at the same time).
Operating the switching elements of the parallel bridge circuits in an interleaved pattern can reduce harmonic content of the power converter output. However, differential mode chokes and other components can be necessary to reduce current imbalance which can result from interleaved control of the parallel bridge circuits. Operating parallel bridge circuits according to a non-interleaved switching pattern can still result in current imbalance among the parallel bridge circuits, particularly during transients (e.g. switching). A number of factors can lead to this imbalance. For example, driver circuits used to drive the bridge circuits can contain opt isolators for isolation of control signals. Each of these optoisolators can provide different delay times in the control signals. Different delay times in the control signals can cause minor differences in the switching times of the switching elements (e.g. IGBTs) used in the bridge circuits.
Typically, at least one inductive element is coupled between the plurality bridge circuit. The inductive element can be a stray line inductance between the parallel bridge circuits or output inductor of the power converter. Any difference in timing between switching of the switching elements can cause a voltage across the at least one inductive element, leading to a circulating current between the parallel bridge circuits. The circulating current can cause a current imbalance between the parallel bridge circuits. The imbalance in current can result in a difference of temperatures in the switching elements used in the parallel bridge circuits, such as a difference in junction temperature of IGBTs used in the switching elements. This reduces the overall output power capability of the power converter as the total output current capability is limited by the switching element with the highest temperature.
Thus, a need exists for a system and method of reducing current imbalance among parallel bridge circuits in a power converter used in power generation systems, such as wind generation systems.